The present invention relates generally to electronic ballasts for powering discharge lamps. More particularly, the present invention relates to electronic ballasts capable of powering one or more lamps independently in both a full illumination (brightness) mode and a dimmed illumination mode that is less than the full illumination mode. Even more particularly, the present invention relates to dimmable electronic ballasts capable of powering multiple lamps independently in a true parallel configuration and providing appropriate lamp filament heating to ensure stable and reliable operation of the lamps in the dimmed mode.
Dimmable electronic ballasts have become increasingly popular because of their light output control and energy saving features. However, conventional dimming ballasts are typically not configured to power multiple discharge lamps independently. Instead, most existing dimmable electronic ballasts are configured for powering lamps connected in series, which means that if any one lamp is removed from the circuit all of the lamps will be shut down.
Further, in such an arrangement if any one lamp reaches an end-of-life condition the ballast itself will very likely be permanently disabled. This necessarily requires lamp replacement after the failure of any one lamp in the serial connection, which can be a significant expense, particularly when repeated over a period of time.
Another drawback for traditional so-called parallel lamp ballasts, typically having a current-fed parallel resonant topology, is that whenever a single lamp is taken out of the circuit the lamp current through the remaining lamps in the circuit may change dramatically. A true parallel lamp configuration should maintain the same lamp current output regardless of the number of lamps coupled to the electronic ballast.